This invention relates to vehicle driving internal combustion engine idle air control apparatus effective to prevent engine stall under engine idle operating conditions while maintaining a low engine idle speed for maximum fuel economy. It is helpful to minimize engine idle speed to improve engine fuel economy; however, the engine is thus operated near its low speed stall limit with the result that a decrease in engine speed due to a sudden load increase or a change in environmental conditions may place the engine in a region of operation in which the engine generated torque is insufficient to overcome the engine load; and the engine stalls.
In the prior art, most engines have been provided with open loop idle air control apparatus which maintained an idle speed sufficiently high that no expected variation in idle speed would be sufficient to stall the engine. Of course, such an engine theoretically wastes fuel at idle since most of the time it need not be operated at such a high idle speed.
A prior art solution which improves engine fuel economy is an idle air control apparatus which includes an engine speed responsive closed loop control to maintain a low engine idle speed but respond to variations in engine idle speed by increasing or decreasing idle air flow as necessary to maintain a substantially constant engine idle speed. Such controls have proved, in some cases, to successfully prevent engine stall while improving engine idle fuel economy, but only with a great deal of difficulty in design and calibration because of the different system gains required under different engine operating conditions.
One of the major difficulties in the design of a closed loop speed responsive idle air control system is the problem of the need for fast response versus stability of the system. An internal combustion engine, particularly one of the multicylinder variety, exhibits speed variations at idle which can be classed in three basic classes. The fastest and largest speed variations are those due to the imposition of a sudden load on the engine such as the initiation of an air conditioning compressor or power steering pump. These speed variations are easily large enough to stall an engine operating near its low speed stall limit and must be corrected by a quick and comparatively large increase in idle air flow. A slower change, but one also capable of stalling an engine, is caused by changes in environmental parameters such as atmospheric air pressure or humidity or engine parameters with wear. These changes must also be corrected, although more slowly. There are, lastly, rapid small random fluctuations in engine speed resulting from the pulses of certain individual cylinder firings and other causes, for which fluctuations it is not necessary to correct, since they are generally not large enough to cause engine stall. However, in some systems, these last variations may be sufficient to cause stability problems in the closed loop engine speed control if that control is provided with a high gain.